This invention relates to a cable data system by which two-way data communications, such as Internet access, is provided via a cable televisions system. In particular, this invention relates to an improved method and apparatus for connecting the physical cable media to a cable modem termination system (CMTS) so as to expedite installation and, among other things, provide a more serviceable CMTS to maintenance personnel.
Cable television systems are well known. In such a system, several different frequency-division multiplexed television channels are distributed to subscribers over a coaxial cable. Each television channel is typically allocated a frequency band, (typically 6 MHz.) in which audio and video information for a television channel is carried. Data signals can also be modulated onto an RF carrier and be transmitted in one or more of the pre-allocated television frequency bands. By allocating one or more T.V. channels for data, the cable television network can readily carry data, such as the data exchanged between computers. Cable data systems provide Internet access to subscribers at speeds that are far greater than dial-up modems.
A cable communications system topology resembles an inverted tree or a directed acyclical graph. The top or upper-most node in a directed acyclical graph (DAG) representing a cable distribution system is the node from which signal information is distributed and is frequently referred to as the cable system head end. Each link in the DAG represents a coaxial cable on which there might be several different frequencydivision multiplexed signals.
One or more cable modem termination systems (CMTS) at or near the head end direct the distribution and collection of data to, and from, cable data system subscribers. At the head end of a cable data system, there are typically hundreds of physical cables that branch out from the head end to and from the system subscribers""homes over a hybrid-fiber coaxial system. Downstream signals are transported on a hybrid-fiber coaxial cable with carrier frequencies centered above the 50 MHz point in the cable spectrum, while upstream signals are transported on a hybrid-fiber coaxial cable with carrier frequencies centered in the 5-42 MHz region of the cable spectrum.
Several different upstream channels can be frequency division multiplexed onto a single cable. In order to recover each channel, the upstream signal must be divided (or split) so that it can be coupled into separate RF band pass filters before being terminated at a unique Physical Interface (PHY) chip. Each PHY chip filters and demodulates the upstream signal for a particular channel to re-create the digital data stream for that channel.
If each upstream channel is transported on a different upstream cable, then a different cable must be used to inject each upstream channel into a unique CMTS upstream connector. In the high-capacity, high-bandwidth CMTS systems of the future, there will be many upstream channels supported by a single CMTS, so this will result in a large number of cables connecting to the CMTS in a relatively small area (yielding very high cable densities). These high cable densities, and the provision of CMTS circuitry to accommodate different system designs, with different numbers of multiplexed channels can be difficult to manage. In order to achieve maximum system flexibility, a CMTS should be able to accommodate different numbers of multiplexed upstream channels on the upstream cables. Even if such capability is provided in a CMTS however, marrying the CMTS to the actual cables has been a serious problem because historically, each upstream channel that is delivered into the CMTS is typically assigned to a unique cable connector on the CMTS. As a result, if N channels are frequency-division-multiplexed on a single upstream cable, then that cable had to be split N times before being connected to the N cable connectors on the CMTS. This splitting is typically performed using splitter circuitry external to the CMTS. After implementing the N splits, a copy of the same signal (with all N of the frequency-division-multiplexed channels) is then injected into each of the N cable connectors on the CMTS. Frequency selective filtering behind each of the N cable connectors select and detects a particular upstream channel out of the N frequency-division-multiplexed channels.
The use of splitter circuitry external to the CMTS results in complicated wiring which adds additional hardware cost and is prone to wiring errors and connector faults. A method and apparatus that integrates this splitter circuitry into the CMTS would be an improvement. Unfortunately, it is not usually known how many unique cables and how many splits will be required at a particular cable TV head-end office. Thus, a method and apparatus by which frequency-division multiplexed signals on different numbers of cables can be easily routed to different numbers of filters would be an improvement over the prior art.
In a cable data system, upstream frequency-division multiplexed signals that are received at a CMTS on a single cable and which need to be divided (i.e. split), by the individual frequency bands, are routed to CMTS channel interface cards that include band pass filters using a relay matrix. Delivering frequency division multiplexed signals to band-pass filters (so that various signals can be selectively retrieved) is accomplished using a computer-controlled input switch matrix of relays arranged to provide different signal division factors. In the preferred embodiment, the relay matrix is configured to accept any number of input signals (up to eight (8)) and route each of the input signals to as many as eight (8) different pass band filters. Alternate embodiments can route the input signals on a cable to virtually any other number of band pass filter inputs, depending upon the number of signals that are multiplexed together on a particular cable.